It's hard to judge if this is a bug (given users who've experienced this as such) or intentional.

Someone in that thread says Youtube confirmed they were doing this but when finding a source[1] statement Google don't directly acknowledge this but use language about having launched an effort to urge viewers to allow ads or buy Premium, whatever they may mean by that.

[1] https://www.androidauthority.com/youtube-long-unskippable-ad...